Die casting machines for ladeling molten material such as non-ferrous metals into a mold are typified by U.S. Pat. Nos. 3,652,073--D. S. Lewis--Mar. 28, 1972 and 3,430,685--H. Drugowitsch--Mar. 4, 1969. These machines have a piston displacement pump plunger that meters out a predetermined charge of molten metal into a cold chamber die.
Because the molten material is displaced by the plunger, the quantity of the charge delivered is very sensitive to the surface level to the molten material. It has therefore not been feasible to use the displacement plunger under conditions where the melting pot level varies. Thus, the charges delivered are not maintained within the close limits required to fill the die without waste over a considerable number of die cycles taken from the melting pot reservoir before refilling or readjusting the level.
An attempt to cure this problem is found in U.S. Pat. No. 3,591,052--A. Nef--July 6, 1971, wherein a large volume reservoir of molten metal feeds a smaller volume pump disposed therein. The level of the reservoir is adjusted when it falls below an index level by insertion of a displacement body to compensate for the molten metal being pumped into the die.
Not only are time delays introduced by this technique, and the necessity to avoid surface ripples or waves, but it also is applicable only when the production speed is slow enough to let the system equalize. Also the degree of control of a large reservoir cannot be as precise as for the control of the smaller quantities in the pumping system and thus the accuracy or precision of a delivered charge can still vary considerably.
Primarily this technique is unacceptable because the amount of level adjustment is dependent upon a level sensor and surface level relationship that can vary differently than the actual change in level caused by removing a charge.
It is therefore an object of this invention to provide improved precise controls of the size of the charge displaced out of a reservoir of molten material into the cold chamber of the die casting machine, by accurately controlling the level of the material in the reservoir as a function of the amount of charge delivered rather than the level of the material in the reservoir.
Also, the Nef compensating system is complex in requiring two separate control systems and two controlled plungers.
It is an object of this invention to provide a simpler and therefore more reliable control means.
Further objects, features and advantages of the invention will be found throughout the following description, drawing and claims.